As the storage density of magnetic recording hard disks has increased, there has been a corresponding reduction in the magnetization-remanence-thickness product (Mrt), and a corresponding increase in the coercivity (Hc), of the magnetic recording layer. Mrt is the product of the recording layer thickness t and the remanent (zero applied magnetic field) magnetization Mr (where Mr is measured in units of magnetic moment per unit volume of ferromagnetic material) of the recording layer. Hc is related to the short-time switching field or intrinsic coercivity (H0) required by the disk drive write head to write data on the recording layer. The trends in Mrt and Hc have led to a decrease in the ratio Mrt/Hc.
To achieve the reduction in Mrt, the thickness t of the magnetic layer can be reduced, but only to a limit because the stored magnetic information in the layer will be more likely to decay. This decay of the magnetization has been attributed to thermal activation of small magnetic grains (the superparamagnetic effect). The thermal stability of a magnetic grain is to a large extent determined by KUV, where KU is the magnetic anisotropy constant of the layer and V is the volume of the magnetic grain. As the layer thickness is decreased, V decreases. If the layer thickness is too thin, KUV becomes too small and the stored magnetic information will no longer be stable at normal disk drive operating conditions.
One approach to the solution of this problem is to move to a higher anisotropy material (higher KU). However, the increase in KU is limited by the point where the coercivity Hc, which is approximately equal to KU/MS (MS=saturation magnetization), becomes too great to be written by a conventional recording head. A similar approach is to reduce the MS of the magnetic layer for a fixed layer thickness, which will reduce Mr since Mr is related to MS, but this is also limited by the coercivity that can be written.
U.S. Pat. No. 6,280,813, assigned to the same assignee as this application, describes a magnetic recording medium wherein the magnetic recording layer is at least two ferromagnetic layers antiferromagnetically-coupled together across a nonferromagnetic spacer layer. In this type of magnetic media, referred to as AFC media, the magnetic moments of the two antiferromagnetically-coupled layers are oriented antiparallel in remanence, with the result that the net or composite Mrt of the recording layer is the difference between the Mrt of the upper and lower ferromagnetic layers. The upper ferromagnetic layer typically has a higher Mrt than the lower ferromagnetic layer so that the composite Mrt is given by MrtUL−MrtLL. This reduction in Mrt is accomplished without a reduction in volume V. Therefore the thermal stability of the recording medium is not reduced.
AFC media thus significantly improve the performance of magnetic recording disks. A low composite-Mrt means a low value of PW50, which is the half-amplitude pulse-width of an isolated read-back pulse of the recorded signal measured at low recording density. The PW50 value determines the achievable linear density, and a low value of PW50 is desirable. Therefore, the extendibility of AFC media is mainly determined by how much the structure can be used to reduce PW50, and this is determined by how large a value Mrt can be achieved in the lower ferromagnetic layer, since MrtCOMPOSITE=(MrtUL−MrtLL).
However, with current AFC media there is a maximum Mrt value that can be used in the lower ferromagnetic layer. Above this maximum Mrt, the media's intrinsic signal-to-noise ratio (S0NR) (the ratio of the isolated signal pulse to noise at a specific recording density), will become worse even though PW50 is still lowered and the composite Mrt is still dropping. For example, an AFC structure can be fabricated with a thicker lower ferromagnetic layer (increasing the lower layer Mrt by 0.05 memu/cm2 above the maximum value) to achieve a PW50 value reduced by 3.5% from the reference AFC structure using the maximum lower layer Mrt. However, this results in an unacceptable decrease in S0NR of approximately 3.5 db. This is shown in FIG. 1.
There are two likely reasons why this decrease in S0NR occurs with AFC media when the lower layer becomes too thick. First, as the lower ferromagnetic layer is made thicker, its anisotropy-volume product (KUV) increases. The KUV determines how susceptible the layer is to thermal fluctuations with the higher the KUV the less susceptible is the layer. It is well established that it is thermally-activated reversal that allows the small interlayer exchange field in AFC media to reverse the magnetization of the lower layer and thereby produce the desired antiparallel remanent configuration. Therefore, the higher the KUV of the lower layer (the higher the lower layer Mrt), the more difficult it is for the relatively small exchange field to completely reverse the magnetization of the lower layer. Second, the magnitude of the exchange field is inversely proportional to the lower layer Mrt, also making it more difficult for the antiferromagnetic interaction to reverse the magnetization of the lower layer as it becomes thicker. Therefore, as the lower layer Mrt is increased there are two effects that occur that make it more difficult to reverse the lower layer magnetization to form the antiparallel remanent configuration. These factors could cause some lower layer grains to not be antiparallel with their respective upper layer grains, possibly producing extra noise in the recorded signal causing the drop in S0NR that is measured. Increasing the exchange field by adding a high moment layer adjacent to the Ru layer is a potential way of postponing this problem, but the addition of this high moment layer reduces the S0NR such that in practice it is very difficult to significantly alter the exchange field without reducing S0NR. Therefore, the problem of not being able to increase the thickness of the lower ferromagnetic layer in AFC media above some maximum value is a universal problem with these structures.
What is needed is a magnetic recording disk with an AFC structure that can take advantage of the reduction in composite Mrt and PW50, but without causing a reduction in S0NR.